1. Field of the Invention
The present invention relates to a hot-dip galvanized steel sheet with excellent spot weldability and stability of material properties. More particularly, the invention relates to a high-strength hot-dip galvanized steel sheet with excellent spot weldability and stability of material properties, including tensile strength (TS), elongation (total elongation, EL), and yield strength (YP), regardless of conditions of a cooling process after annealing (soaking) the steel sheet, variations in these properties being very few, in a high range of the tensile strengths (TS) from 780 to 1180 MPa.
2. Description of the Related Art
Recently, there have been increasing demands for improvement in collision safety performance of vehicles or the like. High-strength steel sheets are widely employed in frames of a vehicle body and the like so as to ensure the passenger's safety on collision, and to improve fuel economy by reducing an increase in the vehicle weight, which brings about by attachment of a fail-safe device. In particular, in order to prevent a part of the bent frame from entering a cabin of the vehicle when it is hit from the side, high-tensile-strength steel sheets with an extremely high tensile strength of about 780 to about 1180 MPa are used. Among them, a composite structural (or dual phase, which is abbreviated to “DP”) steel sheet, which consists essentially of ferrite and martensite, are often used for multipurpose applications because of both excellent strength and ductility. Since steel sheets for the vehicle require excellent capability of corrosion prevention, a hot-dip galvanized steel sheet having a composite structure, and a galvannelaed steel sheet which is obtained by applying an alloying procedure to the hot-dip galvanized steel sheet, have been developed as the steel sheets with both these properties (for example, see JP-A No. 198459/1989, JP-A NO. 105960/1993 and JP-A No. 193419/1999).
Any one of the documents above discloses that a high-strength hot-dip galvanized steel sheet with excellent formability and the like is produced by optimizing manufacturing conditions of a continuous hot-dip galvanizing line using steel whose chemical composition is controlled.
“Properties of 590 MPa grade low YP type hot-dip galvannealed steel sheet”, December 2002, R & D KOBE STEEL ENGINEERING REPORTS, vol. 52 No. 3, by Yoshinobu Oomiya et al. discloses a hot-dip galvanized steel sheet with a tensile strength level of 590 MPa, and not of 780 to 1180 MPa, which has enhanced formability and spot weldability by transforming a three-phase structure including ferrite, martensite, and bainite into a complete composite structure composed of ferrite and martensite by compositional addition of small amounts of Cr and Mo.
It is well known that composite structural or dual phase steel sheets consisting essentially of ferrite and martensite vary greatly in material properties (which mean mechanical properties of steel sheets, more particularly, tensile strength, total elongation, and yield strength in the invention), depending on the conditions of the cooling process (cooling rate, and cooling hold temperature) after annealing (soaking) the steel sheet. Generally, hot-dip galvanized steel sheets (and further hot-dip galvannealed steel sheets) are produced by pickling a hot-rolled steel sheet, cold rolling the pickled sheet to form a cold-rolled steel sheet, and then performing hot-dip galvanizing (and further alloying) of the cold-rolled steel sheet in a continuous hot-dip galvanizing line. In the continuous hot-dip galvanizing line, an annealing (soaking) process is performed in a continuous annealing furnace, a cooling process is performed until the annealed steel is cooled to a temperature for the hot-dip galvanizing after the annealing, and then a galvanizing process is performed. In the cooling step among them, austenite is normally transformed into a rigid structure including martensite, bainite, and the like, by forced cooling means, such as gas cooling, mist cooling, or roll cooling which involves bringing the steel sheet into a contact with a cooled roll skid. Thus, though the cooling rate and cooling termination temperature must be strictly controlled to obtain a desired composite structural steel sheet, it is very difficult to constantly perform and control the cooling on certain conditions on an actual manufacturing floor for various reasons. The thus-obtained products vary greatly in material properties, disadvantageously resulting in a problem that cracks and the like occur due to variations in dimensional accuracy in press forming.
Accordingly, a hot-dip galvanized steel sheet which exhibits high strength ranging from about 780 to 1180 MPa is required to be provided which has not only excellent inherent spot weldability, but also excellent stability of material properties regardless of manufacturing conditions (in particular, the cooling process of the steel sheet after annealing), variations in the material properties being very few. None of JP-A No. 198459/1989, JP-A NO. 105960/1993 and JP-A No. 193419/1999, however, discloses the steel sheets manufactured for such a purpose. Thus, the steel sheets disclosed therein have insufficient stability of material properties. It should be noted that since “Properties of 590 MPa grade low YP type hot-dip galvannealed steel sheet” above fails to take into consideration a range of strengths from about 780 to 1180 MPa, as distinct from the invention, this document basically differs from the invention in the idea of chemical composition design for the purpose of obtaining the desired properties (as described later).